1. Field of the Invention
The present invention relates to a method and apparatus for manufacturing a vitreous silica crucible used to pull up silicon single crystal, and more particularly, to a method and apparatus for manufacturing a vitreous silica crucible having excellent inner surface properties.
2. Description of Related Art
A Czochralski method (CZ method) using a vitreous silica crucible is employed in the manufacture of silicon single crystal. Such a method includes immersing a seed crystal in a silicon melt in the vitreous silica crucible and manufacturing a single crystal by gradually pulling up the seed crystal, at a high temperature, wherein a high purity vitreous silica crucible for holding the silicon melt is used.
The vitreous silica crucible used to pull up single crystalline silicon is mainly manufactured by using an arc fusing method. The method (rotation molding method) is used to manufacture the vitreous silica crucible by forming silica powder molded body due to depositing a raw material powder, such as silica or the like, to a predetermined thickness on an inner surface of a rotation mold formed of carbon or metal, and vitrifying the silica powder by heating and fusing the silica powder due to an arc discharge of an electrode installed at an inner upper part of the mold.
The vitreous silica crucible has a two-layered structure consisting of an outer layer (bubble layer) including at least a plurality of bubbles, and a transparent inner layer (transparent layer). A method of fusing the silica powder while depressurizing and removing gas from the silica powder molded body by sucking up the silica powder molded body from the mold side so as to remove bubbles of an interior to be a glass layer is known as a method of manufacturing such a vitreous silica crucible (JP-A-Nos hei 6-191986, and hei 10-025184). In order to depressurize and remove gas from the silica powder molded body (this is called vacuum suction) during the fusing process, a vacuum level inside the silica powder molded body is increased by forming a thin glass layer (hereinafter, referred to as a seal layer) on a surface by uniformly and thinly fusing the inner surface of the silica powder molded body, thereby sealing the inner surface of the silica powder molded body.
Also, recently, based on requests for efficient device processes, or the like, a diameter of a manufactured wafer has increased to exceed 300 mm, and thus a pulling up time has increased. When the pulling up time increases such that a temperature exceeds 1400° C., a vitreous silica crucible capable of pulling up a single crystal having a large diameter by enduring about 100 hours is required. Also, based on demand for minute devices, or the like, properties of a crucible, such as an inner surface state of the vitreous silica crucible, which directly affect properties of a pulled up single crystal, such as controllability of an oxygen index (Oi), a single crystallization rate, or the like, strongly need to be improved.